Monitoring the rotational characteristics of a rotatable shaft with high accuracy has been very difficult to achieve in the past, and yet is extremely useful in many applications. Prior art shaft position encoders using optical, electrical (impedance) or mechanical angular position sensing arrangements are useful for obtaining average shaft rotation velocity measurements. Unfortunately, such arrangements are generally of limited capability in measuring instantaneous shaft velocity accurately. In addition, most of these techniques cannot be used to measure the velocity of very slowly rotating shafts without the introduction of considerable complexity and attendant measurement error.
For example, one commonly-used prior art shaft position encoder arrangement includes a ring or disk mounted on the shaft, the ring defining equally spaced apertures about its circumference. Light produced by a light source on one side of the ring is sensed by an optical sensor on the other side of the ring. As the shaft rotates, the ring alternately passes and occludes light travelling along the path between the light source and the light sensor. Average shaft rotational velocity can be determined by various methods, such as timing the periods between receipt of adjacent light pulses or counting the pulse rate.
An important disadvantage of this type of prior art rotational sensing arrangement is that its accuracy is inherently limited by the number of apertures defined by the ring and by the accuracy of the (ideally uniform) spacings between apertures. Moreover, this type of measuring arrangement has a resolution limit relative to the measurement of the instantaneous angular velocity of the shaft which is determined by the aperture spacing. As this is a sampling process, the number of apertures per unit time determines the maximum sampling frequency, and thus there is a rather well defined frequency limit to the variations which can be detected. The average rotation speed can be sensed by means of a counter, but spectral analysis and/or filtering are required to determine the presence of speed variations--and the maximum detectable frequency of such speed variations is limited to a frequency of one-half the pulse rate (aperture frequency) or less.
Hence, although optical encoding transducers are capable of producing reasonably accurate macroscopic measurements of average shaft velocity, they are inherently limited in resolution and frequency response in their ability to respond to instantaneous variations in shaft velocity over small angular displacements--such as variations caused by shaft imbalance, shaft loading, shaft torsion, and gear cogging.
Continuous shaft position transducers are generally known. For example, the following (by no means exhaustive) list of prior-issued U.S. patents disclose prior art relevant to capacitive shaft rotation transducers:
U.S. Pat. No. 3,702,467 Melnyk (1972) PA0 U.S. Pat. No. 4,350,981 Tanaka et al (1982) PA0 U.S. Pat. No. 4,364,046 Ogasawara et al (1982) PA0 U.S. Pat. No. 4,410,852 Guretzky (1983) PA0 U.S. Pat. No. 4,477,810 Tanaka et al (1984) PA0 U.S. Pat. No. 4,482,859 Fournier (1984) PA0 U.S. Pat. No. 4,499,465 Tanaka et al (1985)
The typical shaft transducer disclosed in the patents listed above includes a variable capacitor coupled to a rotating shaft, the capacitance of the variable capacitor varying in response to rotation of the shaft. Typically, the prior art utilizes the changing impedance of the variable capacitor rotational transducer as a measure of shaft rotational position. For example, the variable capacitor may be connected to an alternating current (AC) oscillator circuit which generates an alternating current signal the frequency of which depends on transducer capacitance (much as the tuning frequency of a common table radio receiver changes with the capacitance of a variable tuning capacitor). The oscillator frequency changes as the capacitor shaft rotates. Various techniques (most of which require complex frequency demodulation circuitry) are used to extract positional information as well as velocity information derived from rate of change of position from the resulting alternating current signal.
See also Bryner et al, "Sliding Capacitive Displacement Transducer", NASA Tech Briefs, (February 1987), which discloses a linear displacement sensor circuit using a tubular variable capacitor transducer.
While such prior art arrangements are useful in some applications, they generally have the disadvantage that the variable-frequency AC signal cannot be used directly to indicate shaft rotational speed accurately, but must instead be processed further (often with complex circuitry such as balanced modulators or ring demodulators which are capable of determining frequency changes) to derive a signal which indicates shaft speed. As a result, the prior art shaft rotational measuring systems are expensive, sometimes unreliable, and are often of limited accuracy due to errors such as those introduced by the frequency dependence inherent in AC frequency-determining or demodulating circuitry.
Readout circuits for capacitive transducers using DC-operated and AC-operated charge amplifiers are generally known. For example, Tobey et al, Operations Amplifiers: Design and Applications, McGraw-Hill (1971) describes (at pages 233-35) DC-operated charge amplifiers for capacitive transducers. Wolffenbuttel et al, "Capacitance-to-Phase Angle Conversion for the Detection of Extremely Small Capacities," IEEE Transactions On Instrumentation and Measurement, Vol. IM-36, No. 4, pp. 868-872 (December 1987) describes capacitive transducer readout using an AC-operated charge amplifier. These arrangements have been used for sensing pressure, displacement, touch and acceleration. However, so far as I am aware, such techniques have not in the past been used to measure shaft rotation (and instantaneous shaft velocity).
In contrast to the arrangements described above, the present invention provides a signal proportional to true instantaneous shaft velocity. This signal provides information about all rotational vibrations of a shaft which may be caused by any device causing the shaft to move or producing variations in its motion. Examples of sources of vibration are the characteristics of gear teeth meshing together in a gear box, loose particles in lubricants, faulty bearings, and commutators on motors.
The present invention provides shaft rotation sensing which is highly reliable, requires no complex frequency generating or detecting circuitry, has no inherent low frequency limitation, and can easily have a practical upper frequency limit in excess of 50,000 Hz (e.g., 3,000,000 RPM). The signals produced by a sensor in accordance with the present invention are quite sensitive to true instantaneous shaft movements--so that a "signature" of a rotating shaft can be determined. This signature contains unique characteristics indicating such things as very small instantaneous speed variations due to gear tooth meshing, motor commutator contacting cogging, and/or shaft imbalance.
By providing accurate, direct measurements of virtually all shaft speed variations, the instantaneous shaft rotation analyzer provided by the present invention helps determine extremely useful information about shaft and associated driving machine operation (e.g., need for lubrication; failure of bearings, commutators, windings or the like; and variable shaft loading).
In one embodiment of the present invention, a rotatable shaft is coupled to a rotatable, continuously-variable capacitor. A constant voltage is applied across the capacitor rotor and stator plates. The capacitance between the capacitor rotor and stator plates changes linearly with shaft rotation over an increment of rotation so that the current flow to/from the capacitor is a direct measure of the time rate of change of the capacitance (i.e., rotational velocity).
This resultant current from the varying capacitor "current source" is converted to an output voltage, e.g., by means of a transresistance (or transimpedance) amplifier circuit. The output signal includes a superposition of:
(a) a time-varying signal proportional to the instantaneous time rate of change of capacitance (and therefore instantaneous shaft rotational velocity--that is, shaft position change with respect to time), and PA1 (b) a periodic signal (e.g., a square wave) having a frequency related to the frequency of rotation and the number of capacitor poles, as well as having a peak-to-peak amplitude which is proportional to average rotational velocity (or speed).
Various arrangements may be used to eliminate the periodic signal component while retaining all useful signal data.
There are a number of applications where useful diagnostic information is somewhat shaft-synchronous and can be obtained from a time-domain trace. For example, the signature obtained from the commutator on d.c. motors of all sizes can be used to analyze motor performance. A similar type of data can be obtained from reciprocating engines of all types, as the torque pulse produced by each cylinder will generate a signature having an amplitude-time trace which permits a detailed diagnosis of the loading symmetry. Even the nature of combustion may be examined due to the instantaneous velocity characteristics of the variable capacitance tachometer.
Some types of data are not necessarily synchronous, such as those associated with gear boxes and belts--which do not necessarily run at integral multiples of the shaft speed nor are even necessarily related to the shaft speed by ratios of small integers. However, signature information of this type can be recorded and observed in the frequency domain by means of spectral analysis of the data with the Fast Fourier Transform (FFT) or other frequency domain analysis techniques. In this case, it may be important that spectral magnitudes be averaged, so that contributions from asynchronous data would be cumulative.
Another useful application for the sensor in accordance with the present invention is in permanent installations where the sensor can be used for both monitoring and control (and not merely to obtain diagnostic information). In these applications, the present invention has three distinct advantages over other types of tachometers: (1) increased sensitivity, (2) increased frequency range, and (3) introduction of minimal extraneous data due to fundamental linearity of the sensor with respect to rotational speed and position.